Fluid-operated braking systems have long been used to control the movement of motor vehicles in a safe and effective manner. In particular, air brakes are commonly used on commercial vehicles such as trucks, which typically have large gross vehicle weights. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component which is instrumental in the operation of air brake systems is the brake actuator, which provides the force necessary when braking a vehicle.
In a typical double diaphragm spring brake actuator, a barrel-shaped power spring stores potential energy and exerts the large force required for braking in case of air pressure failure. Air pressure acting on a diaphragm compresses the power spring and maintains it in its brake release position. When the air is exhausted, the power spring acts on the diaphragm, typically an elastomeric diaphragm or a piston, and thereby applies the brakes in case of failure of the system air pressure.
The power spring is positioned in a spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and a flange case (sometimes known as an adaptor). The power spring is compressed within the spring chamber between the head and the diaphragm. The power spring has a high spring constant and is normally compressed to a height of less than 3 inches from an original uncompressed height of from 9 to 12 inches. The power spring, therefore, stores a substantial amount of potential energy, usually exerting a force on the head of about 2,000 to 3,000 pounds.
Unauthorized removal of the head, therefore, could be dangerous due to the large potential energy of the power spring. In addition, unauthorized replacement of the head could result in a poor diaphragm seal or other internal defects. Therefore, it is conventional to discourage unauthorized removal and replacement of the head by attaching a warning label and/or providing a securing means which is tamper-resistant or at least tamper-evident, such that a new securing means, not readily available to unauthorized personnel is required before the head can be reattached.
Various approaches have been heretofore proposed for securing the brake actuator head to the flange case to prevent improvident disassembly of the two parts. GB Patent No. 2,000,225, for example, discloses a brake actuator including a head secured to a flange case with a circlip. The circlip is seated in an inwardly facing groove of the flange case and extends over an annular lip of the head, such that the head can only be separated from the flange case by deforming or destroying the flange case. The circlip, however, is accessible from between the flange case and an edge of the head and, therefore, could possibly be tampered with.
The '225 patent also discloses a brake actuator including a head having a lower edge that is spun, or inelastically deformed, over a flange case, such that the head can only be separated from the flange case by deforming or destroying the lower edge of the head. The inelastically deformed edge, however, is accessible and, therefore, could possibly be tampered with.
U.S. Pat. No. 4,850,263 discloses a brake actuator including a head having an annular lip spun, or inelastically deformed, over a flange of a flange case, such that the head can only be separated from the flange case by deforming or destroying the lip and thus the head. The inelastically deformed annular lip, however, is accessible and, therefore, could possibly be tampered with.
U.S. Pat. No. 5,285,716 discloses a brake actuator having a head and a flange case welded together in a tamper-proof manner. As disclosed in the patent, however, since the weld is accessible it is a relatively easy task to grind away the weld to remove the head. In addition, the flange case must be made of steel in order to be welded to the steel head. Flange cases are normally made of aluminum.
U.S. Pat. No. 5,315,918 discloses a brake actuator including a head screwed onto a flange case and secured with an insert or a pin creating an interference fit. The insert or pin may be welded in place, but again, both are accessible and, therefore, could possibly be tampered with.
What is still needed, therefore, is a tamper-resistant spring brake actuator having a head secured to a flange case such that unauthorized removal of the head is discouraged, prevented and evidenced. Preferably, means employed to secure the head to the flange case will be substantially inaccessible to further reduce the possibility of removing the head from the flange case.